Musnad Ahmad 25:1464

Waki’ narrated to us, al-A’mash narrated to us, from Habib ibn Abi Thabit, from Sa’id ibn Jubayr, from Ibn Abbas, who said: "The Messenger of Allah (may Allah bless him and grant him peace) combined the Dhuhr and the ‘Asr, and the Maghrib and the ‘Isha, in Medina, without [the presence of] fear or rain." I said to Ibn Abbas: "Why did he do that?" He said: "So that his Ummah would not be put to hardship."